Image sensors are used in many different types of electronic devices to capture an image. For example, consumer devices such as video cameras and digital cameras as well as numerous scientific applications use image sensors to capture an image. An image sensor is comprised of photosensitive elements that sense incident illumination and produces an electrical signal indicative of that illumination. Each photosensitive element is typically referred to as a picture element or pixel.
Image sensors include charge coupled devices (CCD) and complementary metal oxide semiconductor (CMOS) sensors. Imaging sensors may be capable of capturing grayscale and/or color images. Imaging sensors capable of capturing color images often employ a color filter array (CFA) to separate visible illumination of different color. For example, each of the pixels can be covered with a red, green, or blue filter according to a specific pattern. For example, the Bayer pattern has a repeating pattern of an alternating row of green and red and an alternating row of blue and green. As a result of the filtering, each pixel of the color image captured by a CMOS sensor with CFA responds to only the illumination of wavelengths determined by the color filter of the three primary light colors.
CFA color filters are typically transparent to infrared illumination. In video and still digital cameras, an infrared-cutoff filter is typically placed in front of the sensor to ensure that only visible wavelength illumination reaches the sensor. This is done to make possible accurate color reproduction in captured images.
White balance is the compensation of an image for variation in color temperature of scene illuminant. Images are adjusted in such a way that gray objects will look gray when displayed on a standard output device with a predefined color temperature. Other colors displayed on the output device will also appear as they appeared during image capture. Without white balance, images captured using illuminants having a low temperature will exhibit a reddish cast when displayed on a standard RGB monitor with the color temperature of the white point of 6500K. Illuminants of higher color temperature can create a bluish cast. The human eye can compensate for the different color temperatures. Both film and digital cameras, however, need to correct for different light sources in order to render an image in its true colors.
In a film camera, the color correction can be accomplished manually by the camera user attaching a color correction filter over the lens. A digital camera can correct the color temperature automatically by estimating the color temperature of the scene illuminant. Then the entire image is adjusted by the difference between the scene illuminant color temperature and the color temperature of the white point of the target output device (e.g., a standard display). One problem with this approach is that the white balance algorithm has to be informed as to the correct color temperature of the scene illuminant.
Various techniques exist in the art to determine the color temperature and spectral content of the illuminant. The color temperature of the illuminant can be reliably identified if one knows which objects in the scene are white. Determining the current lighting conditions in a digital camera may be difficult since cameras cannot reliably automatically identify which objects in the scene are white and which are not and thus must rely on other estimation techniques. However, illumination sources often vary with respect to how much infrared radiation they emit. Having an estimate of how much infrared radiation is present in the scene compared to visible illumination can provide information about the nature of the illuminant. For example, incandescent lamps emit visible light accompanied with strong infrared content. Office fluorescent lighting typically emits very little infrared when compared to scene luminance. Outdoor daylight contains moderate amounts of infrared compared to scene luminance. For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for detecting ambient infrared in a sensor equipped with an IR cut-off filter.